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Mr. Stacey 04-11-04 08:11 AM

What makes a bike coast?
I know close to nothing about bicycles.

I have a midrange performance folding Dahon (20x1.5) and a hybrid GIANT with racing tires (700x25c). I was riding with my friend and he was using my GIANT while I was using the Dahon. His coast (no pedaling) is equal to my pedaling. I know a 20" Dahon is no match for a performance bike BUT what accounts for the BETTER coasting?

I road the GIANT this morning and it coasted very very far. Blocks and blocks!

AndrewP 04-11-04 08:50 AM

Low rolling resistance of the tires is the main thing. Friction in the hubs will have some influence. The angle off vertical from the front of the road contact patch to the centre of the hub, is in effect the grade of the hill the bike is trying to roll up. The small diameter wheel gives a larger angle than the large wheel for the same length contact patch. The wider tire on the Dahon will shorten the contact patch but if it is being inflated to a lower pressure that will increase the rolling resistance. A 25 mm tire will have a lower rolling resistnce than a 23 mm wide tire if they are inflated to the same pressure.

Magna Man 04-11-04 10:33 PM

Gravity, i think.

55/Rad 04-11-04 10:52 PM


slvoid 04-12-04 04:09 AM

Momentum, inertia, he's got much larger thinner tires too. Which not only have less resistance but also the fact that your tiny tires have to make more rotations to equal the distance covered by 1 rotation of his, meaning you encounter more friction. Also his bike's larger and has more mass.
I know when I'm riding with my gf on my mtb and she's on her Giant hybrid, even though I carry more and I'm heavier and slightly more aerodynamic, we both go down the hill at around the same speed since she's got 700x35 tires and I've got 26"x2" tires.

caloso 04-12-04 05:04 AM

Newton's 1st Law of Motion explains why a bike coasts: I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Your friend's bike has less friction at the road (the external force) and thus continues in that state of motion longer.

deliriou5 04-12-04 06:00 AM

nope not inertia.... well inertia explains why both bikes will coast at all, but it is friction that determines which will coast longer.

deliriou5 04-12-04 06:02 AM

and yes, larger diameter wheels will roll longer, not because the smaller tires have to make more rotations for a given distance, but because the contact patch is much smaller with the bigger wheel.

geebee 04-12-04 06:12 AM

The hybrid will obviously have less resistance for the reasons stated above , but it might be worth going over your folder and check hubs aren't overtightened, wheels true, brakes not binding, tire pressure, etc.
I just had to change my rear tube and discovered my rear hub was to tight (not noticable against the free wheel friction) when I went looking most bearings were just a bit to tight and it was recently serviced at lbs, these things add up when coasting or going for top speed :)

slvoid 04-12-04 06:53 AM


Originally Posted by caloso
Newton's 1st Law of Motion explains why a bike coasts: I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

I=mr^2. If the other wheel is twice as massive, inertia doubles. If the other wheel is the same mass but twice the diameter, the inertia increases by 4. Etc. This also explains why a smaller wheel has more acceleration, less inertia to overcome when starting.

commuteORdie 04-12-04 07:48 AM

it's because God has blessed your GIant bike and not your Dahon ;)

Don Cook 04-12-04 01:29 PM

You and bicycle in motion have a property called "momentum" (mass x velocity). According to the laws of conservation you and your bike will continue your motion forever, unless acted upon by some outside force. Why did you and the Dahon seem to so much more effected by outside forces that your friend and the Giant? Lets say you and your friend were traveling in the same direction at the same speed. And then decided to coast. You begin your coasting at the same moment. To start things off, the heavier combination of rider and bike will have the greater momentum. But then those pesky outside forces start robbing you both of speed. Air drag, and the frictional forces such tyre air pressure, tyre width, wheel hub bearing friction, cassette freewheel ratchet friction, will bring you both to a stop. Whichever the two of you comes to a stop first, had either the lowest initial momentum, the greatest amount of outside forces acting against you, or a combination of both.
I just think this needs to be said regards 700cx23 and 700cx25 tyres. The 23s are not neccessarily "faster" than 25s. In my application of 700cx25s on one of my bikes, they have less rolling resistence than do a pair of 700c23s on another bike of mine. Why? Because I have gotten used to riding with about 120psi pressure regardless of wether I'm riding 23s or 25s. A wider tyre has less rolling resistence than does a norrower tyre inflated to the same pressure. So if you don't like pumping your 23s up to 135-145- psi and prefer something a little more comfy (120-130psi) you can cut you rolling resistence by going to a 25 and inflating it to 120-130psi.

chumpslacker 04-13-04 06:20 PM

lazy biker's.

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